This invention relates to optical fibers and in particular, sheathed optical fibers and a method for their installation.
A method of installing optical fiber transmission lines is disclosed in European patent application No. 83306636.8 (Title: Optical Fiber Transmission Lines, Applicants: British Telecommunications, Inventors; M. H. Reeve and S. A. Cassidy and corresponding to U.S. Pat. No. 4,691,896, which patent is hereby expressly incorporated by reference), which utilizes the drag forces generated by gas flow to propel optical fiber transmission lines through tubular installation pathways, for insertion or withdrawal of such lines. The optical fiber transmission lines usually comprise optical fiber members in which one or more optical fibers are enclosed in a common jacket.
It is an object of the present invention to provide optical fiber members especially suited for use with the aforesaid method of installation. The method of installation as set forth in U.S. Pat. No. 4,691,896 is described in detail herein.
Optical fiber cables carrying optical fiber transmission lines have heretofore been installed by the same methods as conventional metal conductor cables, those methods usually involving pulling the cable with a pulling rope through a previously laid cable duct. Frequently, the cable duct already contains one or more conventional cables at the time of installing the optical fiber cable.
Unlike the metal conductors of a conventional cable, the optical fibers are easily damaged by tensile stress. Such stress may, for example, propagate micro-cracks, leading to fiber breakage in the long term. It is, therefore, standard practice to reinforce optical fiber cables by providing a central strength member, usually one or more steel tension wires, about which the optical fibers are disposed. The strength member takes up, and thus increases the ability of the cable to withstand, tensile stresses accompanying installation of the cable.
Unfortunately, the central strength member usually provides insufficient protection against local stresses caused by pulling a further cable through the same duct. The conventional approach of installing at the outset optical fiber cables containing sufficiently large numbers of optical fibers to satisfy foreseeable future traffic demands is a way of overcoming this problem. In consequence, first time installation of optical fiber cables containing dozens or even hundreds of optical fibers are currently envisaged despite the fact that to begin with a small fraction of the installed fibers would provide ample traffic carrying capacity. A further reason for installing optical fiber cables of comparatively large dimension is that the smaller the cross-section of the cable the more prone the cable becomes to wedging in between those cables already present in the duct.
The first time installation of large diameter optical fiber cables with high numbers of optical fibers, is, however, undesirable for a variety of reasons. Firstly, there are problems of a technical nature inherent in such cables, such as for example, the difficulty of forming joints and of achieving the required high strength-to-weight ratios. Secondly, there are clear economical drawbacks in committing large resources to install initially unused fiber capacity, particularly in view of the comparatively recent origins of optical fiber technology which lead one to expect continued substantial reductions in the price and improvement in the quality of optical fibers. Thirdly, there is the serious risk of damaging in a single incident very large numbers of expensive optical fibers and, finally, there is an appreciable loss in flexibility when routing high density optical fiber transmission lines.
A method of installing optical fibers with pulling ropes and pull chords is described in "Sub-ducts: The Answer to Honolulu's Growing Pains", Herman S. L. Hu and Ronald T. Miyahara, Telephony, 7 Apr. 1980, pp 23 to 35. The installation method described there proceeds as follows: A section of existing 4-inch (100 mm) duct is rodded and thereafter between one and three individual 1-inch (25 mm) polyethylene tubes are inserted into the duct using pulling ropes. The polyethylene tubes form sub-ducts into which an optical fiber cable can be pulled with the aid of a nylon pull chord which has previously been inserted into the sub-duct by means of a parachute attached to its leading end and pushed through the subduct with compressed air.
The method just referred to does deal with some of the problems discussed above, but only to a very limited extent. Thus, it enables fiber capacity to be increased in up to three stages, and separates the optical fiber cables from those cables already in the duct, thereby greatly reducing the likelihood of jamming, and hence overstressing, of the optical fiber cable.
It is an object of the present invention to overcome, or at least appreciably mitigate the majority of the aforementioned problems of installing optical fiber transmission lines.
It is another object to provide a method of installing optical fiber transmission lines which is comparatively simple and yet flexible and economical.
According to the present invention a method of advancing a lightweight and flexible optical fiber member along a tubular pathway comprises propelling the fiber member along the pathway by fluid drag of a gaseous medium passed through the pathway in the desired direction of advance.
It will be appreciated that to generate sufficient fluid drag to propel the fiber member, the gaseous medium has to be passed through the pathway with a flow velocity much higher than the desired rate of advance.
The terms "lightweight and flexible" in respect of the optical fiber member are to be understood as meaning "sufficiently lightweight and flexible" for the fiber member to be propelled by the fluid drag.
Whether the fiber member is sufficiently lightweight and flexible and the flow velocity sufficiently high is readily determinable by a simple trial and error experiment, guided, if necessary, by the theoretical model discussed below.
The flow velocity of the gaseous medium may be steady or may be suitably varied, for example either between a first velocity producing no, or insufficient, fluid drag to propel the fiber member, and a second velocity producing sufficient fluid drag to propel the fiber member, or between a first and a second velocity both producing sufficient fluid drag for propelling the fiber member. Conveniently, the variations in velocity take the form of repeated abrupt changes between the first and second velocity.
The aforementioned variations in flow velocity may include periods during which the flow is reversed with respect to the desired direction of advance of the fiber member.
It is to be understood that more than one fiber member may be propelled along the same tubular pathway.
A fiber member may, for example, comprise a single optical fiber, protected by at least a primary coating but preferably contained within an outer envelope. Alternatively, a fiber member may comprise a plurality of optical fibers contained within a common envelope.
The envelope may be loosely or tightly surrounding the fiber or fibers.
The method may be used for insertion of an optical fiber member into, or its withdrawal, from the pathway.
The gaseous medium is chosen to be compatible with the environment in which the invention is performed, and in ordinary environments will be a non-hazardous gas or gas mixture.
With the proviso about compatibility with the environment, the gaseous medium is preferably air or nitrogen.
The tubular pathways and/or the fiber members are conveniently but not necessarily of circular cross-section, and the fiber member is always smaller than the pathway.
In practice the pathway internal diameter will generally be greater, and frequently much greater than 1 mm, and the external diameter of the fiber member greater than 0.5 mm.
A preferred range of diameters for the pathway is 1 to 10 mm, conveniently between 3 and 7 mm, and a preferred range of diameters for the fiber members is 1 to 4 mm, although much larger diameters may be used provided the fiber member is sufficiently lightweight and flexible. The diameter of the fiber members is preferably chosen to be greater than one tenth, and conveniently to be about one half of the pathway diameter of greater (and appropriately less, or course, if more than one fiber member is to be propelled through the same pathway).
Insertion of a fiber member by means of the fluid drag of a gas passing over the fiber member has several advantages over methods involving pulling an optical fiber cable with a pull cord.
Firstly, the extra step of providing a pull cord is eliminated.
Secondly, using the fluid drag of a gaseous medium produces a distributed pulling force on the fiber member. This is particularly advantageous if the installation route contains one or more bends. If, as would be the case with a pulling cord, the pulling force were concentrated at the leading end of the fiber member, any deviation of the pathway from a straight line would greatly increase friction between the fiber member and the internal walls of the pathway, and only a few bends would be sufficient to cause locking the fiber member. The distributed pulling force produced by the fluid drag, on the other hand, enables bends to be negotiated fairly easily, and the number of bends in a given installation is no longer of much significance.
Thirdly, the fluid drag substantially reduces overall pulling stress on the fiber member and so permits the fiber member to be of relatively simple and cheap construction.
Furthermore, because the fiber member is not subjected to any substantial pulling stress during installation, little allowance, if any, needs to be made for subsequent relaxation.
According to a further aspect of the present invention, a method of installing an optical fiber transmission line comprises installing a conduit having one or more ductlets providing tubular pathways and, after installation of the conduit, inserting by the aforesaid method using fluid drag one or more fiber members into the associated ductlets as required.
Installing optical fiber transmission lines by this method has several advantage over conventional techniques.
Firstly, since the conduit is installed without containing any optical fibers, conventional rope pulling and similar techniques may be freely employed for installing the conduit.
Secondly, the capacity of a transmission line can readily be adapted to requirements. Thus, while initially only one or two fiber members may be sufficient to carry the traffic the conduit may contain a much larger number of ductlets than are required at the time of installation, and further fiber members may be inserted later on as and when needed. The conduit of the present invention is cheap compared to the cost of the fibers, and spare ductlets to accommodate further fibers as and when extra capacity is required can thus be readily incorporated without adding more than a small fraction to overall costs.
The method of the present invention also permits the installation of improved later generations of optical fiber transmission lines. It is possible, for example, to install at first one or more fiber members incorporating multimode fibers, and at a later date add, or replace the installed multimode fiber members with fiber members incorporating monomode fibers. Installed fiber member may conveniently be withdrawn from the ductlet, and replacement fiber members be inserted by using the aforesaid method of propelling by fluid drag of a gaseous medium.
According to yet another aspect of the present invention, an optical fiber cable comprises a conduit including one or more ductlets forming tubular pathways and capable of loosely accommodating an optical fiber member, and an optical fiber member, and at least one optical fiber member inserted by the aforementioned method using fluid drag. The conduit may be rigid or flexible.
Where the conduit includes more than one ductlet, the ductlets are conveniently formed by bores in the material of the conduit. The term "bore", like the word "tubular" is understood in this context to include circular and other suitable shapes of cross-sectional area.
Alternatively, the conduit may comprise a plurality of individual tubes enveloped by a common outer sheath.
It will be appreciated that the present invention largely avoids the risk, inherent in handling optical fiber cables with a large number of fibers, of accidentally damaging before or during installation in a single event a large number of expensive optical fibers.
The present invention also enables the installation of continuous optical fibers over several installation lengths without joints.
Furthermore, individual fiber members routed through the conduit can be routed, without requiring fiber joints, into different branch conduits at junction points.
According to the present invention, an optical fiber member which is particularly suited for the aforementioned method of installation includes a sheath comprising an inner sheath containing one or more optical fibers, and an outer sheath containing the inner sheath, wherein the inner sheath comprises material of a first elasticity modulus, wherein the outer sheath comprises material having a second elasticity modulus and low density, and wherein the first modulus is high as compared to the second modulus.
The outer sheath is conveniently directly adhered to the inner sheath.
The inner sheath may be in the form of a matrix of sheathing material containing the fiber or fibers. Alternatively, the inner sheath may comprise a sleeve surrounding the fiber or fibers.
The inner sheath may loosely surround the fiber or fibers, but preferably forms a tightly fitting envelope to the fiber or fibers.
The inner sheath may comprise a coating applied to the optical fiber or fibers. Alternatively, the inner sheath may be formed by extrusion about the fiber or fibers.
The outer sheath layer is conveniently formed by extrusion about the inner sheath.
The outer sheath suitably comprises cellular material of low density and is preferably of a substantially greater cross-sectional area than the inner sheath.
The material of the outer sheath preferably has an elasticity modulus of between 10.sup.7 and 10.sup.8 Nm.sup.-2.
In a preferred form of the present invention the sheath comprises an inner sheath in the form of a thin annular sleeve of relatively high density polymer, and an annular outer sheath enclosing the inner sheath and formed of relatively low density foamed polymer. Where the sleeve contains a plurality of optical fibers, the sleeve conveniently fits sufficiently tightly for the enclosed optical fibers to be closely packed.
While conventionally constructed fiber members have been used successfully for installation by the technique disclosed in U.S. Pat. No. 4,691,896, the applicants have found that by employing fiber members according to the present invention improvements such as, for example, greater continuous installation lengths, reduced likelihood of damage to the optical fiber or fibers, etc, can be achieved. In the following detailed description, initially these fiber members are described followed by a description of the above-described method of installation.